1. Field of the Invention
The present invention relates to a semiconductor strain sensor for detecting acceleration, pressure or the like.
2. Description of the Related Art
When stress is applied to a semiconductor element, such as silicon, germanium, its resistivity changes. This phenomenon is called the piezoresistance effect. Both pressure sensors and acceleration sensors use the piezoresistance effect.
A conventional semiconductor strain sensor is disclosed in U.S. Pat. No. 4,967,597 and uses silicon for its piezoresistance effect. Silicon has many advantages over other materials--the piezoresistivity of silicon is large, it is mechanically strong, and many techniques for manufacturing devices using silicon have been established.
However, the practical piezoresistivity of silicon is limited at approximately 50.times.10.sup.-12 cm.sup.2 /dyn because it cannot self-compensate for temperature when its piezoresistivity is higher than that. Therefore, it has been difficult to use a silicon-based semiconductor strain sensor to detect a very low acceleration or pressure.
The technical report of the Institute of Electronics, Information and Communication engineers, ED88-18, P51-56 (1988) "Piezoresistance Effect in Selectively Doped Structure" by Kato et al., proposed a strain resistive element having a hetero structure of Al.sub.x G.sub.al-x A.sub.s /G.sub.a A.sub.s. The strain resistive element has a high sensitivity. However, this strain sensor uses G.sub.a A.sub.s structure on which the hetero structure is formed. Therefore, the strain sensor has not been practically used due to weak mechanical strength of G.sub.a A.sub.s.